Coal

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Coal lumps are harvested directly from coal ore blocks, which must be destroyed using any level of pickaxe. After this, they cannot be reverted back into block form.
They are the most efficient fuel available for furnaces, being able to smelt eight units of ore per lump of coal. Coal lumps and charcoal are also the only fuels accepted by powered minecarts (providing approximately four minutes of transit each).
The only purpose for which coal could be considered required is the creation of torches